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New Scientist Live

Floating feeling

I have a clear childhood memory of sitting in the back of a car as it was driven over an arched bridge, and feeling an odd sensation, known colloquially as “butterflies”, in my stomach, similar to the one you feel on a fairground wheel. I remember that the adults in the front of the car felt nothing. Now, as an adult, I no longer experience this. Is this feeling, which my contemporary friends also recall, age-related or are cars manufactured differently? Or is something else at work?

• The feeling is a reduction of apparent gravity, due to the downward acceleration as the car passed over the hump of a bridge. The back seat of the car is usually positioned over the back wheels – in some old cars even further back – which maximises the effect. The front seats, on the other hand, are about half way between front and back wheels, which reduces the effect.

While the questioner’s age may have something to do with not feeling butterflies any more, so too might the fact that adults usually sit in the front. And then there’s the age of the car, modern suspensions smooth out the bumps much more than the old ones did.

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Tim Gossling, Cambridge, UK

• This phenomenon was known in my family as “leaving your tummy behind” and its occurs far less these days because modern roads have been levelled off, so there are very few good take-off points left.

I can, however, highly recommend a stretch of road on the A38 near Lichfield where the dual carriageway passes over a canal bridge. The road engineers have tried their best to remove the fun, but my children used to scream with delight when this was taken at a legal speed that was fast enough to cause momentary freefall of car and occupants. Adults notice it less because in the front of the car you can see it coming. The kids in the back can’t. And you certainly don’t tell them…

Richard Jack, Lichfield, Staffordshire, UK

• The feeling of disorientation is caused by the brain’s confusion over the motion that the inner ear senses but the eye does not see.

Adults can see out of the car windows while children normally end up staring at the back of the front seats. When the car passes over a humpback bridge, the fluid in the inner ear that detects movement senses the rise and fall of the car, yet the child’s eyes are fixed on a stationary seat in front of them. It is this mismatch of sensory input that causes the sensation of butterflies in the stomach.

This is exactly what causes sea sickness too. Everything on the boat seems stationary to the eye, but the inner ear is sensing a rise and fall as you pass through the waves.